The present invention relates to communications systems and more particularly to systems and methods for power control in communication systems that employ orthogonal frequency division multiplexing or OFDM.
A point to multipoint wireless communication system represents a potentially effective solution to the problem of providing broadband network connectivity to a large number of geographically distributed points. Unlike optical fiber, DSL, and cable modems, there is no need to either construct a new wired infrastructure or substantially modify a wired infrastructure that has been constructed for a different purpose.
In order to conserve scarce spectrum, the data communication devices of a point to multipoint wireless communication system may share access to a common frequency. In a typical scenario one or more frequency channels are allocated to downstream broadcast communication from a central access point to a plurality of subscriber units and one or more separate frequency channels are allocated to upstream communication from the subscriber units to the central access point. For upstream communication, there is a medium access control (MAC) protocol which schedules subscriber unit transmissions to prevent interference.
For a given upstream frequency, the time domain is divided into frames which are typically of equal duration. Each frame represents an individually allocable unit in the time domain. One subscriber unit transmits in each frame. Reservations for transmission in a particular frame are made by the central access point and distributed in broadcast downstream transmissions in accordance with the MAC protocol.
In such a point to multipoint wireless communication system, it is generally preferable to centrally control the transmission power of each subscriber unit. Each subscriber unit should transmit at a power level high enough to assure accurate reception of the transmission yet not so high as to swamp the front end of the central access point's receiver or cause interference to unintended receivers. Power control involves monitoring subscriber unit transmitted power at the central access point and sending power adjustment information downstream to maintain power at the desired level.
Cable modem systems also have a point to multipoint architecture, also involve access to a shared medium, and also require subscriber unit power control. It would be desirable to simply adopt a MAC protocol already developed for cable applications to the wireless context. One such protocol that has been developed is referred to as the MCNS protocol. The MCNS protocol is described in Data-Over-Cable Service Interface Specifications, Radio Frequency Interface Specification, SP-RFI-I04-980724, (Cable Television Laboratories, 1997), the contents of which are herein incorporated by reference.
A cable MAC protocol like MCNS is already implemented in low cost chip sets. The operational characteristics of MCNS are well known. Hardware and software to implement higher layer protocols is designed to interoperate with MCNS. Furthermore, it is desirable to maintain parts commonality between wireless modems and cable modems to the extent possible.
The MCNS protocol provides for controlling the power of subscriber units from a head end. In the MCNS protocol, subscriber units may be requested by the head end to transmit upstream for the purpose of measuring their power. The head end responds with a downstream power adjustment command. Each upstream power measurement transmission by a subscriber unit requires use of a MAC frame that is then unavailable for data transmission by other subscriber units. Unfortunately, a wireless communication system requires more frequent updates of subscriber unit power level than does a cable modem system. Using the power control features of MCNS to provide sufficiently frequent updates in a wireless system consumes valuable capacity. Furthermore, the frequent update operations are burdensome to hardware implementing the MAC protocol.
OFDM (Orthogonal Frequency Division Multiplexing) is a highly useful communication technique. In OFDM, the available bandwidth is divided into subchannels that are orthogonal to one another in the frequency domain. A high data rate signal is effectively transmitted as a set of parallel low data rate signals, each one being carried over a separate subchannel. OFDM addresses a problem known as multipath caused by differences in delay time among different paths taken from a transmitter to a receiver. The effect of multipath is intersymbol interference created by energy associated with different symbols sharing a common arrival time. By creating multiple low data rate subchannels, OFDM lengthens the period occupied by a single symbol so that dispersive effects tend to be confined within a single symbol period, thereby reducing intersymbol interference.
It would be desirable to provide power control functionality to a wireless OFDM point to multipoint communication system and to exploit existing cable modem system MAC protocols to the extent possible.